1. Field of the Invention
The present invention relate to a plasma display panel provided with electrodes having a structure bringing about an improvement in the contrast of light emission on the display screen.
2. Description of the Related Art
FIG. 3 illustrates the structure of a plane-emission plasma display panel according to a conventional technique.
In FIG. 3, there are provided stripe-shaped discharge-sustaining electrodes X.sub.n and Y.sub.n formed on a front glass plate 1. Although discharge sustaining electrodes are disposed in the order . . . , X.sub.n, Y.sub.n, X.sub.n+1, Y.sub.n+1, . . . in the specific example shown in FIG. 3, they may also be disposed in the order such as . . . , Y.sub.n, X.sub.n, Y.sub.n+1, X.sub.n+1, . . . to achieve the same functions. The discharge sustaining electrodes X.sub.n and Y.sub.n each comprise a transparent electrode 2 and a bus electrode 3 for supplying electric power to the transparent electrode 2. The discharge sustaining electrodes X.sub.n and Y.sub.n are covered with a dielectric layer 4 on which there is provided a cathode film 5 made up of a MgO film serving as a discharging cathode.
On the cathode film 5, there are provided partition walls 7 extending in a direction perpendicular to the discharge sustaining electrodes X.sub.n and Y.sub.n so that discharging spaces 6 are isolated from each other by the partition walls 7. Address electrodes 8 for selecting a light emission region in the discharging spaces 6 are formed between adjacent partition walls 7 in such a manner that the address electrodes 8 extend in a direction parallel to the partition walls 7. Each discharging spaces 6 are filled with a mixed gas of Ne and Xe.
Furthermore, three-color phosphors 9 are disposed periodically in the order red 9R, green 9G and blue 9B on the surfaces, on the sides facing the respective discharging spaces, of the partition walls and the address electrodes 8. Furthermore, as shown in FIG. 3, there is also provided a back side glass plate 10 on the partition walls and the address electrodes 8.
An nth scanning line is formed by the discharge sustaining electrodes X.sub.n and Y.sub.n. In the discharging spaces 6, a discharging cell in which a discharge occurs is formed at each intersection of the scanning lines and the address electrodes. That is, the plasma display panel has the structure in which discharging cells are disposed in a matrix fashion.
FIG. 4 illustrates a cross section of the conventional plasma display panel, taken along a plane perpendicular to the scanning lines. For simplicity, the partition walls 7, the address electrodes, 8, the phosphors 9R, 9G, and 9B, and the back side glass plate 10 are not shown in FIG. 4.
In FIG. 4, by way of example, dimensions typical for a 40 inch VGA-type plasma display panel are also shown, wherein the values are expressed in .mu.m. As shown in FIG. 4, the nth scanning line is located at the center between a pair of discharge sustaining electrodes X.sub.n and Y.sub.n.
The operation of the conventional plasma display panel is described below.
FIG. 5 illustrates an example of the manner in which a frame is divided into a plurality of fields so that a color image with 256 halftone levels is represented therein.
In this example, one main frame consists of eight subfields (first SF to eighth SF). Each subfield consists of a first priming period (I), a second priming period (II), a small-width erasing period (III), a writing period (IV), and a discharge sustaining period (V).
In any subfield, the periods I-IV are equal in length of time. However, the discharge sustaining period (V) varies in accordance with the rank defined for each subfield. The discharge sustaining period (V) of the (N+1)th subfield is about twice that of the nth subfield (wherein N is a natural number). In the writing period (IV) of each subfield, a desired cell is selected by applying a pulse-shaped voltage to a corresponding address electrode 8. During the following discharge sustaining period (V), a sustaining discharge occurs as many times as the number of sustaining pulses applied during the discharge sustaining period (V). Therefore, the length of the discharge sustaining period (V) is proportional to the number of sustaining pulses.
As a result, the intensity of light emission which occurs in the cell selected in the writing period (IV) increases about twice at each transition from any subfield to the following subfield. In the main frame, the respective subfields are either selected or not selected so as to achieve any arbitrary halftone level selected from 2.sup.8 =256 levels.
FIG. 6 is a timing chart illustrating an example of a sequence of pulses applied, in each subfield, to the address electrodes (W electrodes) and the discharge sustaining electrodes X.sub.n and Y.sub.n.
After passing through the first priming period (I) and further the second priming period (II), a priming discharge occurs between the discharge sustaining electrodes X.sub.n and Y.sub.n in all discharging cells. In the subsequent small-width erasing period (III), an erasing discharge occurs between the discharge sustaining electrodes X.sub.n and Y.sub.n. thereby removing most of charges present on the surface of the cathode film 5 at locations above the discharge sustaining electrodes X.sub.n and Y.sub.n. As a result, information stored in the discharging cell selected in the previous subfield is reset.
In the following writing period (IV), the voltage of the discharge sustaining electrode Y.sub.n is swung from one scanning line to another. In synchronization therewith, a selected/non-selected image signal is applied to the W electrode of the respective cells so that a writing discharge occurs between Xn and Y.sub.n in the selected cells. In the discharging cells in which the writing discharge has occurred, a sustaining discharge occurs as many times as the number of sustaining pulses applied to the discharge sustaining electrodes X.sub.n and Y.sub.n during the following discharge sustaining period (V). On the other hand, in the discharging cells which were not selected in the writing period (IV), no sustaining discharge occurs during the sustaining period (V). By properly selecting discharging cells in the manner described above, a desired image is produced.
In the plasma display panel, the discharging cells which were not selected during the writing period (IV) have no discharge during the discharge sustaining period (V) as described above, and thus black is displayed in these non-selected discharging cells.
The image becomes sharper and clearer with the ratio (light emission contrast) of the light emission intensity (maximum brightness) of the discharge in the discharging cells selected in the writing period (IV) to the black-level intensity of the discharging cells which were not selected in the writing period (IV). In other words, to improve the image quality of the plasma display panel, it is required to increase the light emission contrast.
One possible way of increasing the light emission contrast is to generally increase the number of sustaining pulses applied during the discharge sustaining period (V) thereby increasing the maximum light emission intensity thus increasing the contrast. However, this technique results in an increase in the power dissipation and also an increase in the amount of heat generated in the plasma display panel. Therefore, this technique has a limitation in the maximum light emission contrast.
In the operating sequence described above, "black" is displayed by not selecting the discharging cell of interest during the writing period (IV) in any subfield. However, even in the discharging cells which were not selected during the writing period (IV), a priming discharge and erasing discharge still occur in the first and second priming periods (I, II) and the erasing period (III), respectively. This makes it difficult to decrease the black-level intensity.
Furthermore, as shown in FIG. 4, the priming discharge and the erasing discharge during the first and second priming periods (I, II) and the small-width erasing period (III) occur over the entire width of the discharge sustaining electrodes X.sub.n and Y.sub.n as in the case of the sustaining discharge during the discharge sustaining period (V). In particular, in the second priming period (II) and the small-width erasing period (III), pulses with a large amplitude are applied between the discharge sustaining electrodes X.sub.n and Y.sub.n, and thus the intensity of light emission generated by one discharge during these periods is generally greater than the intensity of light emission generated by one discharge in the discharge sustaining period.
For the above reason, the maximum light emission contrast practically achieved in the conventional plasma display panel is about 50:1. However, the light emission contrast of 50:1 is not high enough to represent fine difference in the light emission intensity in the low halftone-level range. As described above, the conventional plasma display panel has the problem that it is difficult to achieve a high enough light emission contrast without causing an increase in the power dissipation or an increase in heat generated in the plasma display panel.
Thus, it is an object of the present invention to provide a plasma display device having an improved light emission contrast without encountering a significant increase in the power dissipation and heat generation.